A frequency synthesizer includes a Voltage Controlled Oscillator (VCO) for generating various frequencies through voltage control and a Phase Locked Loop (PLL) for locking a frequency generated in the voltage controlled oscillator with a feed-back loop to provide improved frequency stability.
The voltage controlled oscillator uses a LC oscillation method in which an oscillation frequency is determined mainly by the inductance of a coil and the capacitance of a capacitor. When the values of the inductor and the capacitor are fixed in the voltage controlled oscillator using the LC oscillation method, an oscillation frequency is changed by changing only input voltage, a very large VCO gain is required, and power consumption and phase noise increase. Therefore, there is suggested a method for changing a resonant frequency by controlling a capacitance of a voltage controlled oscillator based on a channel code. Such a voltage controlled oscillator includes a capacitor bank array having a plurality of capacitors and a plurality of switches and operates according to a binary weighted array method for changing a total capacitance by controlling the plurality of switches according to an input channel code.
In the voltage controlled oscillator using the binary weighted array, the total capacitance which is changed by the channel code needs to have linearity. That is, when the channel code of the voltage controlled oscillator increases uniformly, a capacitance sum corresponding to each channel code need to increase at uniform intervals. However, intervals between total capacitances may be non-uniform due to process variation and/or a fringe capacitance. As a result, this leads to a situation in which frequency steps of oscillation frequencies of the voltage controlled oscillator become non-uniform.
For example, when the value of a channel code input to the voltage controlled oscillator increases gradually, the value of the oscillation frequency of the voltage controlled oscillator needs to increase gradually. However, when the value of the channel code increases gradually as illustrated in FIG. 1, the frequency steps of frequencies corresponding to specific channel codes may become narrow or there may occur an inversion phenomenon in which the frequency decreases when the channel code increases. For example, when a channel code 31 is changed to a channel code 32, there occurs an inversion phenomenon in which a frequency oscillated according to the channel code 31 is lower than a frequency oscillated according to the channel code 32 as illustrated in FIG. 2. In addition, when a channel code 63 is changed to a channel code 64, a phenomenon may occur which frequencies oscillated according to the two channel codes are identical to each other. When a channel code 95 is changed to a channel code 96, there may occur a phenomenon in which the frequency step between a frequency corresponding to the channel code 95 and a frequency corresponding to the channel code 96 is narrower than a frequency step of frequencies corresponding to other channel codes.
When the frequency step size of frequencies oscillated by the voltage controlled oscillator decreases or there occurs the inversion phenomenon, there is a problem in which a frequency lock time increases which is taken to distinguish a desired frequency among two consecutive frequencies in a frequency synthesizer. In addition, in the binary weighted array, a binary search algorithm is used. When an error occurs during searching, there may occur a situation in which a desired frequency is not searched unlike a liner search method. For example, in a case where the inversion phenomenon occurs when the channel code 31 is changed to the channel code 32 as illustrated in FIG. 2, an incorrect capacitor bank code is accessed during the binary search and eventually a desired frequency may be not searched.
Accordingly, a technology for ensuring linearity of a voltage controlled oscillator in order to improve frequency search efficiency is desired.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.